Exploring the charge carrier dynamics in carbon nanotubes-TiO<inf>2</inf> composites for water photooxidation

Composite materials consisting of stable metal oxides and functionalized carbonaceous nanomaterials are attractive for application in a range of photocatalytic systems. Carbon nanotubes/titanium dioxide (CNTs/TiO2) composites have been shown to outperform TiO2 in photoelectrochemical applications, which has been attributed to efficient electron extraction from TiO2 by CNTs and subsequent fast electron transport. However, the underlying mechanisms need to be clarified. In this work, we present a systematic study varying the CNTs content and comprehensively analyze the composite structure, morphology, crystallinity, light harvesting properties, and photoelectrochemical charge carrier dynamics. We show that the performance improvement observed for the composites depends in a complex manner on several parameters, including the CNTs content, the thickness of the TiO2 overlayer, the chemical capacitance of CNTs, and the hole transfer efficiency. Using intensity-modulated photocurrent spectroscopy in aqueous electrolyte solution both without and with 0.1 M Na2SO3 as hole capturing agent, we show that electron extraction and transport compete with the hole trapping, recombination, and transfer dynamics at the CNTs/TiO2 and TiO2/electrolyte interfaces. We show that the optimal performance is related to the change in charge extraction mechanism from a trap-limited diffusion scenario in TiO2 to a direct charge extraction in CNTs/TiO2 composite with optimal CNTs content and TiO2 thickness. These insights provide valuable guidance for the design and optimization of CNTs/TiO2 and similar composites for photoelectrochemical applications.